1. Field of the Invention
The present invention relates to a light source for testing the characteristics of an optical element used in optical communications, and more particularly to a wide-band or long-band light source for testing an optical element in a fiber amplifier.
2. Description of the Related Art
At present, as the demand for communications is increased, a wavelength division optical communication system has been widely used. Also, researches for a fiber amplifier that can accommodate more channels have been actively made.
In a wavelength division multiplexing (WDM) communication system using multiple channels, the channel spacing generally used is 0.8 nm, and the number of channels is increased to 8, 16, 32, 40, 64, etc. The wavelength band in the range of 1528 nm≈1562 nm, which is the amplification band of the existing erbium doped fiber amplifier (EDFA), can be used until the number of channels reaches 40, but a new wavelength band is required if the channel number is increased over 64.
If the channel spacing is determined to be 0.4 nm in the existing wavelength band, the number of channels that can be used is increased up to 80 channels, but many technical difficulties exist due to a nonlinear phenomenon, etc. Accordingly, research regarding the wavelength band in the range of 1575 nm≈1605 nm, which can be amplified by the erbium doped fiber amplifier (EDFA), have been actively progressed. With this trend, a light source having a wide wavelength band is required for testing various kinds of optical elements used for optical communications, and especially optical elements used for a fiber amplifier. Especially, when using the fiber amplifier in the WDM optical communication system, its wavelength band is in the range of 1520 nm≈1620 nm, and thus a light source capable of accurately testing various kinds of optical elements in this band is required.
FIG. 1 is a view illustrating the construction of a long-band light source using a general EDFA. Referring to FIG. 1, the conventional long-band light source using the fiber amplifier comprises optical isolators 100 and 104 for intercepting a backward propagating light, an erbium doped fiber (EDF) 102 as an amplification medium, and wavelength selective couplers (WSC) 101 and 103 for making pumping lights from pump light sources (i.e., pump laser diodes) 105 and 106 incident to EDF 102. The output characteristics of the long-band light source as constructed above are shown as a dotted curve in FIG. 4. The source output strength of the long-band light source is low.
Known apparatuses, incorporated herein by reference, for controlling the gain of an EDFA are disclosed in U.S. Pat. No. 5,606,724 to Katsuhiro Shimizu et al. entitled Gain Controllable Optical Amplifier And Applications Thereof, and U.S. Pat. No. 5,745,283 to Shinya Inagaki et al. entitled Method And Apparatus For Controlling Optical Amplifier Used For Optically Amplifying Wavelength-Division Multiplexed Signal.
When using a white light source among the conventional light sources, the strength of light output from the white light source is weak, and there exist limitations in testing the performance of optical elements accurately. Also, when using an amplified spontaneous emission (ASE), the difference between the light strengths according to the wavelengths becomes great, and there exist problems in testing an absorption spectrum of the EDF. If the light strengths are greatly different according to the wavelengths when the absorption spectrum is tested by making a weak light signal incident to the EDF, the light strength becomes too high in a specified wavelength, while the light strengths become too low in other wavelengths, and this causes a testing error to occur.
Specifically, at the wavelength having a significant light strength, the light absorbs energy, and excites a longer wavelength. Thus, a pure absorption spectrum cannot be effected at the long wavelength having a lesser light strength. Also, if the light strength is small, it deviates from the testing sensitivity of a spectrum analyzer, and the test itself becomes impossible.
Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and an object of the present invention is to provide a long-band (L-band) light source which has a high light strength and a small source output difference for each wavelength to test the characteristics of optical elements used in a fiber amplifier and optical communications.
It is another object of the present invention to provide a L-band light source which uses a feedback loop to increase the output strength of a L-band in a light source using an ASE.
In order to achieve the above objects, according to the present invention, there is provided a L-band light source for testing optical elements using a feedback loop in a fiber amplifier including a rare earth doped fiber as an amplification medium, forward and backward pump laser diodes, positioned on front and rear ends of the rare earth doped fiber, for generating and providing pumping lights to the rare earth doped fiber, first wavelength selective couplers for providing the pumping lights from the forward and backward pump laser diodes to the rare earth doped fiber, and optical isolators, inserted between front and rear ends of the first wavelength selective couplers, respectively, for intercepting backward lights reflected from input and output terminals of the fiber amplifier, the L-band light source comprising a feedback loop for making a seed beam incident to the rare earth doped fiber or making an amplified spontaneous emission (ASE) incident again to the rare earth doped fiber to reuse the seed beam or the ASE as the L-band light source for testing the optical elements, second wavelength selective couplers, provided between the optical isolators and the first wavelength selective couplers, respectively, for making the seed beam incident to the feedback loop or extracting and providing the ASE to the feedback loop, and a forward optical isolator, connected to the feedback loop, for intercepting a backward propagation of the seed beam or a backward propagation of the ASE.
In another aspect of the present invention, there is provided a L-band light source for testing optical elements using a feedback loop in a fiber amplifier including a rare earth doped fiber as an amplification medium, forward and backward pump laser diodes, positioned on front and rear ends of the rare earth doped fiber, for generating and providing pumping lights to the rare earth doped fiber, first wavelength selective couplers for providing the pumping lights from the forward and backward pump laser diodes to the rare earth doped fiber, and optical isolators, inserted between front and rear ends of the first wavelength selective couplers, respectively, for intercepting backward lights reflected from input and output terminals of the fiber amplifier, the L-band light source comprising a feedback loop for making a seed beam incident to the rare earth doped fiber or making an amplified spontaneous emission (ASE) incident again to the rare earth doped fiber to reuse the seed beam or the ASE as the L-band light source for testing the optical elements, second wavelength selective couplers, provided between the optical isolators and the first wavelength selective couplers, respectively, for making the seed beam incident to the feedback loop or extracting and providing the ASE to the feedback loop, and a forward optical isolator, connected to the feedback loop, for intercepting a backward propagation of the seed beam or the ASE.